Download SEISMIC ASSESSMENT OF BRICK MASONRY BUILDING

SEISMIC ASSESSMENT OF BRICK MASONRY BUILDING
Situated Anywhere In Seismic Zone IV of India
1.
Introduction
If you live in a house constructed using brick walls, without reinforced concrete columns, any where in the
seismic zones IV of India, you should know, for your family’s safety, whether the building will be safe under the next
earthquake or could suffer severe damage. You can yourself assess the level of damageability of your building by filling
a few simple data in the questionnaire framed herein. But before filling the data, you should go through the guideline
provided below for better understanding of your building and the earthquake behavior of such buildings.
2.
Seismic Zone IV and Estimated Maximum Intensity of Future Earthquake
Before using this document, make sure that your building is situated in Seismic Zone IV. For ready reference,
the districts lying in Zone IV, either wholly or with more than 40% area are listed in Table 1. In seismic zone IV a
maximum earthquake Intensity of VIII on MSK Intensity Scale is likely to occur. In terms of building damage, Intensity
VIII is stated to cause the following types of damage:MSK Intensity VIII
Most kutcha buildings constructed using clay walls may be totally destroyed. Most masonry buildings
constructed using brick walls with mud mortar may also be destroyed. But those constructed using good
cement mortar may only have heavy cracking.
Buildings constructed in sandy soil with high water table are liable to more severe damage than in other areas.
Areas of Uttarakhand and many areas in Saurashtra are also placed in Seismic Zone IV where MSK Intensity VIII has
actually occurred during 1991 Uttarkashi earthquake, 1999 Chamoli earthquake and the 2001 Kachchh earthquake
resulting in wide spread loss of life and property due to collapse of masonry buildings.
3.
Basis of Assessment
For the assessment of expected seismic performance of most of the existing load bearing masonry wall
buildings in Zone IV of the Seismic Zoning Map of India, shown in fig. 1, the most direct approach will be to compare
the safety provisions specified in the building code IS: 4326-1993, “Earthquake Resistant Design and Construction of
Buildings – Code of Practice (Second Revision), Bureau of Indian Standards” for such buildings with the condition
actually present in an existing building. Where the existing condition is found to comply with the code, it will be
considered safe and acceptable. But where any existing condition is found deficient, hence non-complying, it will be
considered as weak and damageable. Such a deficiency will require upgradation or strengthening or retrofitting.
Here the residential buildings situated in Seismic Zone IV only are considered.
4.
Type of Building Construction
The following building types can be assessed by the simple procedure covered here:
(i) Brick wall buildings with any mortar and roof type.
(ii) Concrete block wall buildings with any mortar and roof type.
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(iii) Fully dressed stone (Ashler masonry)wall buildings with any mortar and roof type
The number of storeys may be one to four only as five storeyed building are not considered in the Code. The building
may be fully residential or with mixed use. Only earthquake safety aspects are to be assessed. It is assumed that the
building is otherwise safe to carry the vertical weight of roof and floor as well as the use-related imposed loads of
furnishings, partitions, people & the contents like merchandise, office equipments etc.
Table 1:- Districts with more than 40% area falling in Seismic Zone IV
DISTRICT NAME
% AREA
DISTRICT NAME
%
DISTRICT NAME
AREA
BIHAR
%
DISTRICT NAME
% AREA
AREA
HIMACHAL PRADESH
DELHI
Paschim Champaran
100
Lahul & Spiti
96.4
Moradabad
100
North West
100
Purba
100
Solan
96.2
Rampur
100
North
100
Sheohar
92.8
Sirmaur
100
Jyotiba Phule Nagar
100
North East
100
Kishanganj
90.8
Shimla
100
Meerut
100
East
100
Purnia
95.9
Kinnaur
100
Baghpat
100
New Delhi
100
Katihar
100
JAMMU & KASHMIR
Ghaziabad
100
Central
100
Madhepura
46.8
Anantnag
92.5
Gautam Buddha Nagar
100
West
100
Saharsa
55.4
Leh
100
Bulandshahr
100
South West
100
Muzaffarpur
92.9
Kargil
100
Aligarh
56.3
South
100
Gopal Ganj
100
Doda
100
Mathura
49.4
CHANDIGARH (UT)
Chandigarh
Siwan
98.8
Udhampur
100
Pilibhit
77.2
Saran
100
Punch
100
Kheri
49.9
Vaishali
100
Rajuri
100
Bahraich
58.4
Samastipur
100
Jammu
100
Shrawasti
93.3
Begusarai
100
Kathua
87.1
Balrampur
97.4
Khagaria
100
MAHARASHTRA
Siddhartnagar
100
Bhagalpur
100
Raigarh
73.6
Sant Kabir Nagar
43.7
Banka
91.7
Satara
53.6
Maharajganj
100
Munger
100
Ratnagiri
54.1
Gorakhpur
57.9
Sheikhpura
100
PUNJAB
Nalanda
98.2
Gurdaspur
Lakhisarai
100
Patna
88.1
Jamui
73.7
GUJARAT
Bareilly
46.1
100
Kushinagar
100
Amritsar
81.4
Deoria
71.9
Kapurthala
100
UTTARAKHAND
Jalandhar
100
Tehri Garhwal
96.8
Hoshiarpur
100
Dehradun
100
Banas Kantha
42.3
Nawanshahr
100
Garhwal
96.8
Patan
66.1
Rupanagar
100
Champawat
100
Fatehgarh Sahib
98.1
Nainital
100
100
Ludhiana
78.4
Udhan Singh Nagar
100
Ambala
89.1
SIKKIM
Hardwar
100
Yamuna Nagar
100
North
100
WEST BENGAL
HARYANA
Panchkula
Panipat
66.7
South
100
Darjiling
Sonipat
83.1
East
100
Jalpaiguri
83.6
Rohtak
56.2
West
100
Koch Bihar
88.8
Jhajjar
100
UTTAR PRADESH
Uttar Dinajpur
100
Rewari
100
Saharanpur
100
Dakshin Dinajpur
100
Gurgaon
100
Muzaffarnagar
100
North 24 Parganas
64.7
Faridabad
100
Bijnor
100
South 24 Parganas
82.8
100
100
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Fig.1:- Seismic Zoning Map of India
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5.
Factors Considered In Seismic Safety
as Per Building Code IS: 4326-1993
The most important factors considered
in the Code for ensuring seismic safety of
various category buildings are the following:
I) For Safety of Walls
(i) Mortar used in foundations and walls
(ii) Size and placing of door, window
openings in walls
(iii) Length of wall between cross walls
(iv) Height of wall above floor to ceiling
(v) Unconnected perpendicular walls.
(vi) Provision of horizontal seismic bands at
(a) plinth level
(b) door and window lintel level
(c) ceiling of flat floor/roof, or eave level of
sloping roofs
(d) gable ends and top of ridge wall
(e) window sill level
(vii) Provision of vertical steel bar
(a) at each corner/junction of walls
(b) at door & window jambs
II) For Safety of Roofs or Floors
(i) Roofs/floors with prefabricated or precast
elements
(ii) Cantilever balconies
(iii) Roofs/floors with wooden joists with
various covering elements
(iv) Jack arch roof or floors
(v) Sloping roofs with sheets or tile covering
(vi) Sloping raftered roofs
Fig.2:- Essential elements for earthquake safety of
masonry buildings
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6.
•
General guidance about Seismic Safety of masonry Buildings
A single storeyed building using one brick thick walls will be relatively safer than three storeyed one. The fourth
storey, if added, will be unsafe, and will make the lower storeys more vulnerable.
•
Use of half – brick thick (10 to 11.5 cm or 4 to 4½” thickness) load bearing walls will make the storey very unsafe
during seismic Intensity VIII on MSK Intensity scale and, if used in 3rd or 4th storey, it may have a catastrophic
failure.
•
Too many window openings make a wall weaker, and use of smaller size piers less than 18 inches (45 cm) in width
between them will increase the damageability even higher.
•
Richer cement-sand mortar of 1:4 mixture (1 part cement by 4 parts of sand) makes the masonry stronger against
earthquake shaking as compared with 1:6 mortar by a factor of 2.5 to 3.0. Also 1:6 mortar is stronger than limecinder or lime-surkhi mortar.
•
Use of clay mud mortar produces the weakest masonry. Its strength in dry condition reduces to less than 50 percent
when the walls get wet during rains. Hence, use of good plastering is essential to protect such masonry during rainy
months.
•
Longer walls between consecutive cross walls are found weaker than shorter walls. The length is controlled for
safety by limiting its length to thickness ratio.
•
Taller walls between any two floors are found to be weaker than shorter walls. The storey height is controlled by
limiting its height to thickness ratio.
•
All four walls enclosing a room should be properly connected at each corner. Walls not so connected will easily
separate at corners and overturn under the earthquake motion.
•
The most important seismic safety requirement is provision of seismic bands in all storeys in all external as well as
internal walls. These bands maintain the integrity of the whole building as one unit under earthquake shaking.
Besides the earthquake safety, they also increase the stability of the walls under the vertical loads.
•
The roof structure of the sloping roofs needs its integrity through bracing and proper connectivity with the walls.
Such integrity is automatically provided by reinforced concrete slabs wherever used for floors and the roof.
The Indian Standard Code of Practice IS: 4326 – 1993 covers all these aspects in detail and gives specific
guidance as to how to incorporate them in the earthquake safe construction of all types of new masonry buildings.
7.
Form for Seismic Safety Assessment of Housing Units in Masonry Buildings
7.1
Guiding Notes for filling the Assessment Form
(i) Complete all data points. Tick mark columns of complying & non-complying as found by compliance.
(ii) BB stands for Burned Brick Construction and CCB for Cement Concrete Block Construction.
(iii) More than 4 storeys are not permitted by Code (IS: 4326 – 1993).
(iv) The compressive strength of building unit for one and two storey building to be not less than 35 kg/cm2 , and
for 3 & 4 storeys 50 kg/cm2 . Note: Red bricks in the Indo-Gangetic plain are normally strong enough and
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complying. But yellow colored bricks are found weaker, hence, non-complying. CCB strength can be
ascertained from the manufacturers, or determined by testing in laboratory.
(v) Normal bricks in India are 230 mm ( 9 inch) in length and CCB 200 mm wide. Load bearing burnt brick walls
are generally 230 mm thick and those using CCB are 200 mm thick. Smaller wall thickness for non-load
bearing partitions and infill burnt brick walls are kept 115 mm (½ brick) or 100 to 150 mm for CCB.
(vi) Specified mortar mix is C:S = 1:6 or richer in cement-sand.
(vii) Length of wall between cross
walls is to be less than 35 x
thickness or 8 m maximum.
(viii)
Height of wall from
floor to ceiling to be not more
than 15 x thickness nor 4m.
(ix) Following constraints for door
and window openings are
specified for Zone IV (see fig.3)
-
End corner distance b5 =
450 mm
Fig.3:-Opening sizes as per IS: 4326 - 1993
-
Pier width b4 = 560 mm
-
Ratio of overall length of openings in a wall to its overall length, that is (b1 + b2 + b3)/L =
Is to be less than or equal to 0.5 for one storey building, 0.42 for two storey building, 0.33 for 3 & 4 storey
building
7.2
The Assessment Form
Building Information:Address:Location/Area:Village/Town:-
District:-
House Unit under Assessment:Name of Owner/Tenant living in the house:No. of storeys in the building including basement if any:Floor on which the house unit is located: - Ground / First / Second / Third
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FORM FOR ASSESSING SEISMIC SAFETY
S.N
(1)
Data of Building under Assessment
(2)
Required as per Code
(3)
Tick if
Comply
ing
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
Number of storeys, S =
Wall building unit: (Tick one)
BB/CCB (solid)/CCB (Hollow)
Compressive strength =
kg/cm2
Thickness of load bearing walls, t
External wall =
Internal wall =
Mortar used =
Longest wall in room, L = …….m
Height of wall, floor to ceiling
h = ……. m
Door, Window openings (See fig.3)
Overall (b1 + b2+….)/l,
(i)
Give max in any room =
(ii)
b4 min. =
(iii)
b5 min.=
Floor type (tick mark)
a)
RC or RB slab,
b)
Precast RC elements, (tick one)
With/without RC screed
c)
Wood joists with tiles, (tick one)
With/without bracing
d)
Jack arches, (tick one)
With/without ties &bracing
Roof type (tick as present)
a)
Flat like floor (See item 8 a),
b)
Sloping trussed, (tick one)
With/without bracing
c)
Sloping raftered, (tick one)
With/without ties &bracing
Seismic Bands (tick as present)
(i)
at plinth
provided / not provided
(ii)
at lintel level provided / not provided
(iii)
at window
provided / not provided
sill level
(iv)
at ceiling or provided / not provided
eave level
(v)
at gable ends provided / not provided
(vi)
at ridge top provided / not provided
Vertical bar (tick as present)
(i)
at external corners
provided / not provided
(ii)
at external T-junctions
provided / not provided
(iii) at internal corners
provided / not provided
(iv)
at internal T-junctions
provided / not provided
(v)
at jambs of door
provided / not provided
(vi)
at jambs of windows
provided / not provided
Equal to or less than 4
Compressive strength ≥
35 or 50 kg/cm2
BB = 230 mm
CCB = 200 mm
C:S = 1:6 or richer
BB ≤ 8 m
CCB ≤ 7 m
BB = 3.45 m
CCB = 3.0 m
One storeyed 0.50
Two Storeyed 0.42
3 or 4 Storeyed 0.33
b4 min 560 mm
b5 min 450 mm
OK
With RC screed
With bracing
With ties & bracing
OK
With bracing
With ties &bracing
Required
Required
In 3 or 4 storey buildings
only
Required in cases 8 b, c
& 9 b, c.
Required in cases 9 b,c
Required in cases 9 b,c
Required in all masonry
buildings
Not
complying
Action to be taken to save the
building from collapse or
severe damage
1. Extra upper stories may be
removed.
2. For weaker building units,
providing RC or steel columns
may be considered.
3. For thinner walls pilasters of
RC may be designed.
4. For weaker mortar case,
providing Rc columns &
beams may be considered.
Note:- In cases 2, 3 & 4 above,
strengthening the walls by ferrocement plating of by FRP may be
an option.
5 & 6 Provide buttresses at regular
intervals for strengthening
the wall.
7 (i) Consider closing one of the
windows or reduce the width
of window openings by
adding additional thickness
of piers.
(ii) & (iii) strengthening the piers
by ferro-cement plating.
8.(b)Use
RC
screed
with
peripheral RC beam in place
of RC ceiling band
8. (c)Introduce cross ties with
diagonal bracing underneath
the floor.
8. (d)Provide cross ties welded to
the steel joists along with
diagonal bracing.
9(b) Use diagonal bracing in the
plane of the ties and the
principal rafters in every
fourth panel.
9(c) As in 9 (b)
10 (i) Incase the plinth is high say
90 cm or higher provide
seismic belt.
10 (ii) to (vi) Provide seismic belt
on the walls at the level
where seismic band was
required.
11 (i) to (iv) Provide seismic belt
or a vertical bars at the
corners of the walls in the
replacement of the required
vertical reinforcement as
per the code.
11 (v) & (vi) Strengthen the jambs
with seismic belts.
Note: - Record all information for your housing unit and the building of which your unit forms part.
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8.
Deficiencies Vs Type of Damage
Among the assessment points indicating deficiencies in the building, different non-complying elements are
known to create differential damage behavior of a building under MSK Intensity VIII earthquake. Some deficiencies are
critical which lead to the total collapse and some lead to varying degrees of damage. An effort is made below to give
indication of the likely damage scenario in case a masonry building is subjected to Intensity MSK VIII which is the
basis of defining the areas under seismic Zone IV.
a)
Buildings with wall thickness less than that specified in the Code or buildings constructed in weaker mortar, and
those not having any horizontal seismic bands and/or vertical reinforcement at corners and wall junctions can
totally collapse.
b) Those of one or two storeys constructed using specified minimum wall thicknesses, using cement sand mortar of 1:
6 mix and having RC/RB slab floor and roof but without any horizontal seismic bands and or vertical steel bars at
corners, may have wide cracks in the walls particularly in the walls with window openings.
c)
Three or four storeyed buildings with conditions similar to case (b) above may have more severe damage including
partial or total collapse in certain storeys.
d) In buildings having conditions as at (b) above but with longer or higher than specified dimensions of the walls may
also be subjected to severe damage, i.e. partial or total collapse as in case (c) above.
e)
Buildings having Code specified wall dimensions as well as mortar, and reinforced with the horizontal seismic
bands and vertical steel at corners and junctions of walls, will generally remain safe except for minor cracking seen
in the piers between the openings and some walls.
9.
Summary of Results of Comparative Assessment
The major deficiencies indicating non-compliance with Codal provisions should be noted and the house unit
owner may understand the kind of damage to which his unit or the building as a whole may be subjected as explained in
Para. 8 above. Those deficiencies will need to be considered for upgrading the seismic safety by retrofitting the building
suitably to prevent the total or partial collapse of the building in future if and when the probable maximum earthquake
Intensity MSK VIII will strike the area.
10.
References
1.
IS: 4326-1993 "Earthquake Resistant Design and Construction of Buildings - Code of Practice (Second Revision)".
2.
IS: 13935- "Repair and Seismic Strengthening of Masonry Buildings - Guidelines" (First Revision………..).
G o I
–
Pr ep are d B y:
Professor Anand S. Arya and Ankush Agarwal
U N D P , D i s a s t e r R i s k M a n a g e m e n t P r o g r a m m e
E - ma i l: - an an d .s .a r ya @ un dp . or g; a nk us h. a ga rw al@u nd p .o rg
NATIONAL DISASTER MANAGEMENT DIVISION
MINISTRY OF HOME AFFAIRS, NORTH BLOCK, NEW DELHI
Tel:91-11-23093178 ; Tele/f ax : 23094019, Email:nd mind ia@n ic.in ;W eb site:www.nd mind ia.n ic .in
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